The present invention relates to a coulostatic method of evaluating the corrosion rates of metal and a measuring apparatus used for same.
The weight loss method has been long known for evaluating the corrosion rates of metal. This direct method is able to measure the corrosion rate of a metal surely and accurately, but it requires a long time to obtain the measurement result and fails to obtain the corrosion rate change with respect to time.
Recently, the polarization resistance method has been used for electrochemical evaluation of the corrosion rates of metal. The polarization resistance method is disclosed in "J. Electrochem. Soc., 104,56 (1957)" by M. Stern & A. Geary, "Corrosion, 14, 440t (1958)" by M. Stern, and "Proc. Am. Soc. Testing Materials, 59, 1280 (1959)" by M. Stern & E. Weisert. This method is based on the fact that corrosion of a metal involves an electrode reaction where metal ions are dissolved from the metal surface and the rate of the electrode reaction relates to the value of the current flowing in a corrosion reaction. In this method, a metal test piece having a corrosion potential E.sub.corr in a test solution is used as a working electrode. A constant current (I) is then made to flow from the test metal piece to a counter electrode. Under this condition, the potential E.sub.mes is measured. From the corrosion potential E.sub.corr and the measured potential E.sub.mes, the change of the potential .eta. is calculated by 72 = E.sub.mes - E.sub.corr, and then the polarization resistance R.sub.p is obtained by the equation (1) EQU .eta. = I.multidot.R.sub.p ( 1)
The corrosion rate is obtained from the relation that the polarization resistance R.sub.p calculated from the equation (1) in inversely proportional to the corrosion rate.
The polarization resistance method using constant current is rapid for obtaining the corrosion rate, compared to the weight loss method, but it still suffers from the following problems. A relatively long time must be taken until the potential E.sub.mes of the metal piece reaches a constant value, i.e. the potential .eta. reaches a constant value. A long measurement time permits a continuous current flow over the surface of the metal piece, resulting in change of the surface condition of the metal piece. This can cause an experimental errors. In the case that a solution has a large solution resistance, its large ohmic drop gives rise to measurement errors. These errors must be compensated through a complex measuring operation and calculation.